1. Field of the Invention
The present invention relates to an ion generation method and an ion source.
2. Description of the Related Art
One of the standard processes in semiconductor manufacturing is to implant ions into a semiconductor wafer for the purpose of modifying conductivity or modifying the crystal structure of the semiconductor wafer. Apparatus used in the steps are generally called ion implanter.
Direct current (DC) discharge ion sources are known to be used in ion implanter. A DC discharge ion source is configured to heat a filament by a direct current so as to generate thermoelectrons, which heat the cathode. The thermoelectrons generated from the heated cathode are accelerated in an arc chamber and collide with source gas molecules introduced into the arc chamber, thereby ionizing atoms included in the source gas molecules (see patent document 1).
[patent document 1] Japanese patent 3516262
Halides such as fluorides or chlorides are often used as source gas molecules introduced into the arc chamber. The source gas molecules comprising halides generate halogen radicals in the process of ionization. The halogen radicals react with parts used to build an ion source (e.g., a metal member of the inner wall of the arc chamber) and are chemically bonded to the member. The chemically bonded metal member is ionized along with the source gas molecules and is extracted as an ion beam from the ion source.
There is a concern that the ionized metal member of the inner wall of the arc chamber ends up being implanted into the semiconductor wafer, contaminating the wafer with impurity ions such as metal ions. In particular, in the case of a direct current discharge ion source, the arc chamber is heated inside to a high temperature. This is sometimes addressed by making the inner wall of the arc chamber of a high melting point metal such as molybdenum (Mo), tungsten (W), tantalum (Ta), etc. However, such contamination due to a high melting point metal may preferably be reduced as much as possible in order to improve the performance of the semiconductor device.